Since the beginning of the oil industry, large volumes of crude oil and petroleum products have been carried on tankers and barges, usually in single hulled vessels of which a typical longitudinal and mid-ship transversal section is schematically indicated in FIG. 1A and 1B. In the great majority of tankers and barges operating today, the cargo and fuel is still separated from the surrounding water by a single barrier of steel plating which together with the internal structural members are adequate in terms of strength and rigidity for transporting cargo, but offer minimal protection of the environment from cargo and fuel oil spills in case of collision or grounding.
The international maritime community has dealt successfully with the problem of pollution caused by discharging overboard of the ballast water contaminated by oil residue. Until recent times however no provisions existed which were intended to prevent, or minimize, the pollution caused by cargo and fuel oil spills as the result of collision or grounding.
Numerous tanker disasters and ensuing oil spills in recent times however have caught the attention of the world (S.S. Terry Canyon, S.S. Amoco Cadiz, S.S. Corinthos to name just a few) and maritime community regulators have reacted by issuing new rules and regulations aimed at enhancing navigation safety as well as providing some structural protective measures. Thus, so-called "protectively located segregated ballast tank" arrangements for tankers are now mandatory. Unfortunately, all these measures proved inadequate and the need for more stringent regulations became evident in light of the Exxon Valdez and other more recent environmental disasters.
In 1990, the U.S. Congress enacted a law requiring that all new tankers operating in U.S. waters be built with double sides and double bottoms extending the full length of the cargo spaces--the so-called "double-hull" ships. This design in effect interposes two structural barriers separated by voids or spaces (which can also function as ballast tanks) between the cargo of oil and the waterway. FIG. 2A and 2B illustrates a typical double-hull design. U.S. law also now requires that existing single hull vessels, unless modified to suit the required double-hull arrangement, be phased out and scrapped when they reach a predetermined age.
In March 1992, the International Maritime Organization (IMO) endorsed the double-hull concept, and also accepted, as an alternative design, the so-called "Mid-Deck Ship" in which protection against pollution following collision is obtained by means of side (wing) tanks, the same as in the double-hull design, while protection against pollution following grounding is achieved by utilizing the hydrostatic pressure of the water outside the hull to force the oil or other petroleum products in the ruptured tank into another compartment in the tanker. The mid-deck design is illustrated in FIG. 3A and 3B. A description of a mid-deck design is found in A. Hirai, Mid-Deck Tanker: A Proposal for Future Tanker Designs from the Pollution Prevention Point of View, presented at the Philadelphia Section Society of Naval Architects and Marine Engineers, Feb. 19, 1992, the teachings of which are specifically incorporated herein by reference.
The maritime community recognizes that the abovementioned designs do not represent ideal solutions. The double-hull design is complicated structurally and expensive to implement even in newly constructed tankers. It requires the use of considerable amounts of steel plates, beams and pipes, and greatly increases the weight of the vessel and its operating costs. It also incurs other operational and personnel safety problems, namely: cleaning, surveying, and risk of explosion. Furthermore, while the double-hull design appears to be satisfactory in case of low energy grounding, it may even prove detrimental in case of a high energy impact similar to the one sustained by the Exxon Valdez, since when the outer hull is ripped open, the void tends to fill with water, increasing the tanker's draft, thereby causing more petroleum product to spill if the inner hull is also pierced.
Conversely, the mid-deck concept will probably greatly reduce the spill in case of high energy grounding, but will not be very effective in a low energy accident when the cargo is denser than the water or when the cargo will first mix with the water to form a dangerous pollutant. Furthermore, the mid-deck design creates the same expensive steel requirements as the double-hull. The mid-deck design is simple in principle but complicated in reality, has not been tested, and requires serious structural modifications not easily implemented on an existing ship.
Furthermore, for existing tankers, the costs of retrofitting an old vessel with a double hull will be prohibitive, and would therefore force the retirement of such vessels. For these and other reasons, IMO and the U.S. Government have left the door open and will consider any other proposals that offer comparable protection and avoid some of the shortcomings of the two, so far accepted, designs.
Accordingly, it is an object of this invention to provide a new, safer, easy to maintain, more efficient and more economical hull design for waterborne vessels, particularly oil tankers, which will reduce the probability of serious oil or other contaminant spills and environmental pollution as the result of collision or grounding. This new hull design is easily implementable for new vessels and will not significantly increase the overall cost of construction. Furthermore, the new hull design will allow retrofitting of existing single hulls to bring them into compliance with national and international safety and antipollution regulations. Both of these goals are accomplished in an economical manner and without unduly increasing the weight of the vessel. Heretofore, these objectives and advantages have not been satisfactorily achieved with existing or presently proposed hull designs.